Geothermal ground source/water source heat exchange systems typically use fluid-filled closed loops of line buried in the ground, or submerged in a body of water, to either absorb heat from, or to reject heat into, the naturally occurring geothermal mass and/or water surrounding the buried or submerged fluid line. The line loop is extended to the surface and is then used to circulate naturally warmed or cooled fluid to an interior air heat exchange means.
Common and older design geothermal water-source heating/cooling systems typically circulate, via a water pump, a fluid comprised of water, or water with antifreeze, circulated within plastic (typically polyethylene) underground geothermal lines to transfer geothermal heat to or from the ground in a first heat exchange step. Via a second heat exchange step, a refrigerant working fluid heat pump system is utilized to transfer heat to or from the water. Finally, via a third heat exchange step, an interior air handler (comprised of finned line and a fan), or optionally a refrigerant to water heat exchanger, is utilized to transfer heat to or from the refrigerant to heat or cool interior air space.
Newer design geothermal DX heat exchange systems, where the refrigerant fluid lines are placed directly in the sub-surface ground and/or water, typically circulate a refrigerant fluid, such as R-22, R-407C, R-410A, CO2 (R-744), or the like, in sub-surface refrigerant lines, typically comprised of metal (such as copper) lines, to transfer geothermal heat to or from the sub-surface geology via a first heat exchange step. Unlike water-source designs, DX systems only require a second heat exchange step to transfer heat to or from the interior air space, or optionally to interior water, typically by means of an interior air handler or an optional refrigerant to water heat exchanger (all of which are well understood by those skilled in the art). Consequently, DX systems are generally more efficient than water-source systems because less heat exchange steps are required and because no water pump energy expenditure is necessary to circulate the working fluid within sub-surface geology. Further, since metal is a better heat conductor than most plastics, and since the refrigerant fluid circulating within the metal line of a DX system generally has a greater temperature differential with the surrounding sub-surface geology than the water circulating within the plastic line of a water-source system, generally, less excavation and drilling is required, and, consequently, installation costs are typically lower with a DX system than with a water-source system, thereby decreasing payback periods and enhancing economic viability.
While most in-ground/in-water DX heat exchange designs are feasible, various improvements have been developed intended to enhance overall system operational efficiencies. Several such design improvements, particularly in direct expansion/direct exchange geothermal heat pump systems, are taught, for examples, in U.S. Pat. No. 5,623,986 to Wiggs; in U.S. Pat. No. 5,816,314 to Wiggs, et al.; in U.S. Pat. No. 5,946,928 to Wiggs; in U.S. Pat. No. 6,615,601 B1 to Wiggs; and in U.S. Pat. No. 6,932,149 to Wiggs, the disclosures of which are incorporated herein by reference. Such disclosures encompass both horizontally and vertically oriented sub-surface heat geothermal heat exchange means.
While DX systems are generally more efficient than water-source geothermal system designs, DX systems are also generally more efficient than air-source heat pump system designs for several reasons.
One reason is because a DX system does not require a “defrost cycle” while operating in the heating mode. A defrost cycle essentially reverses the air-source heat pump system's reversing valve to direct hot refrigerant gas exiting the compressor into the outdoor heat exchange line (typically finned refrigerant line) to melt any frozen moisture that has accumulated on the exterior refrigerant to air heat exchange lines, to provide full, and/or close to full, airflow and heat exchange across the exterior lines. Unfortunately, when operating in a defrost cycle, valuable warm interior air is utilized to warm and vaporize the system's refrigerant, which warmed refrigerant vapor is then compressed by the system's compressor to materially raise both the pressure and the temperature of the refrigerant. This now hot refrigerant gas is sent outside to melt frozen moisture on the exterior heat exchange line. Therefore, valuable interior heat (in the winter and/or in the heating mode) is typically withdrawn from the interior of a structure and is rejected into the exterior air when the air-source heat pump is operating in a defrost cycle. The resulting loss of interior heat usually must be made up via supplemental or back-up heat, usually comprised of electric resistance heat and/or the burning of a fossil fuel, all of which is very costly and inefficient.
Another reason DX systems are also generally more efficient than air-source heat pump system designs (as well as refrigeration system designs using air as their heat sink) is because, in the cooling mode, waste heat from the refrigerant is being rejected into a relatively cool (often about 50-60 degrees F.) sub-surface environment, as opposed into relatively hot (often about 80-100 degrees F.), exterior air (as would be the case with an air-source heat pump). The typically cooler heat sink for a DX system both keeps refrigerant pressures lower (which directly translates into lower compressor power draws and lower system operational costs) and keeps refrigerant temperatures cooler (which assists in removing more humidity from interior air), with both aspects being advantageous.
Also, a DX system is generally more efficient than an air-source heat pump because no exterior fan is required in a DX system. The total elimination of an exterior fan (designed to blow exterior air over the refrigerant to air heat exchange line of an air-source system) can result in an overall 10-20% system power reduction.
However, while DX systems are more efficient than other various heat pump technology designs, as explained above, a DX system requires a sub-surface heat exchange loop for operation. While such a loop can be placed in water, if an adequate water source/supply is available, most often, DX systems need to be installed with their sub-surface heat exchange loops inserted into drilled wells/boreholes. In such wells/boreholes, the DX heat exchange loops always consist of a hot/warmer vapor phase working fluid (typically a refrigerant) line and a cold/cooler liquid phase working fluid line, which respective vapor line and liquid line are operably coupled together at, or near, the bottom of the well/borehole. Being in close proximity to one another, a concern in any DX system is the potential for “short-circuiting” the advantageous geothermal heat transfer within the well itself, as opposed to primarily and far more advantageously within the interior heat exchanger, because heat naturally flows to cold via Fourier's Law.
Thus, in the heating mode, exiting refrigerant from the well, which has absorbed valuable geothermal heat within the well, loses some portion of the geothermal heat gain to the entering colder liquid fluid line in close proximity within the well, all before the maximum geothermal heat gain possible can be delivered to the compressor for heat accentuation and use, as is all well understood by those skilled in the art. Typically, temperatures might be as low as about 15° F., or colder, within the coldest working fluid within the well, and might exit the well at only around 30° F., or colder, after acquiring geothermal heat. The only protection against such geothermal heat loss from the warmer exiting line to the colder entering line within the well itself is by insulating the cooler liquid line. Currently, such insulation (which is not used at all in water-source systems) in DX system designs is comprised of one of a mostly solid plastic insulation and a mostly expanded foam type rubber or plastic insulation, which insulation surrounds certain portions of the liquid phase line within the well, to help protect against heat transfer from the hot/warmer vapor phase refrigerant transport exiting line to the entering colder at least partially liquid phase refrigerant transport line within the well itself.
In the cooling mode of operation, the “short-circuiting” heat transfer issue/concern within the well is magnified, because of the extremely hot temperatures (typically 140-180° F.) of the vapor line entering the well (with the well and surrounding geology being used as a heat sink) being in close proximity to the return cooler liquid phase refrigerant exiting the well with the exiting refrigerant temperature typically being in the approximately 65-90° F. range. Again, as in the heating mode (the heating mode has a reverse-cycle refrigerant directional flow within the well from that of the cooling mode), the only protection against the “short-circuiting” effect of heat entering the well (via the hot vapor phase refrigerant) being transferred into the exiting geothermal cooled liquid phase refrigerant is the insulation surrounding some portion of the cooler liquid phase line. A normal sub-surface ground temperature range is typically approximately 50-60° F. within about 500 feet of the surface. Thus, if one is able to avoid most of the “short-circuiting” effect between the cooler liquid phase refrigerant line and the warmer vapor phase refrigerant line within the well/borehole itself, overall system operational efficiencies will be increased.